Merge #14839: [rebase] threads: fix unitialized members in sched_param

89282379ba threads: fix unitialized members in sched_param (Cory Fields)

Pull request description:

  Rebased theuni's #14342.

  Building with gcc 8.2 against musl libc, which apparently has more attributes available in its sched_param. The following warnings were produced:

      warning: missing initializer for member 'sched_param::sched_ss_low_priority' [-Wmissing-field-initializers]
      warning: missing initializer for member 'sched_param::sched_ss_repl_period' [-Wmissing-field-initializers]
      warning: missing initializer for member 'sched_param::sched_ss_init_budget' [-Wmissing-field-initializers]
      warning: missing initializer for member 'sched_param::sched_ss_max_repl' [-Wmissing-field-initializers]

  Since the current thread may have interesting non-zero values for these fields, we want to be sure to only change the intended one. Query and modify the current sched_param rather than starting from a zeroed one.

Tree-SHA512: a0bedbcf0130b3ee8261bb704e4bf6c9b760ad377c8a28c258765d54e54462b76707efc188b936b0a635cdd2bdf6b3b9298ab06ba361dc4806150b670d9702a3

src/util/system.cpp

@@ -1290,7 +1290,7 @@ fs::path AbsPathForConfigVal(const fs::path& path, bool net_specific)
 int ScheduleBatchPriority()
 {
 #ifdef SCHED_BATCH
-    const static sched_param param{0};
+    const static sched_param param{};
     if (int ret = pthread_setschedparam(pthread_self(), SCHED_BATCH, &param)) {
         LogPrintf("Failed to pthread_setschedparam: %s\n", strerror(errno));
         return ret;